1. Field of the Invention
This invention relates to a well tool for use in setting other well tools in anchoring or sealing engagement with a conduit in a subterranean well and, more particularly to, wireline setting assemblies used to set well tools, such as bridge plugs, packers and cement retainers.
2. Description of the Prior Art
Well tools, such as bridge plugs, packers and cement retainers used in subterranean oil and gas wells can be anchored or positioned at a subsurface location within the well conduit by a number of means. Some well tools can be set or anchored by means of mechanical manipulation of a tubing string extending from the well tool to the surface or by means of the application of hydraulic pressure through the contiguous tubing string. One common method of setting conventional well tools is by use of a wireline pressure setting assembly. Conventional wireline pressure setting assemblies can be attached to the conventional well tool and run into the well on a conventional wireline unit. When the well tool reaches the desired subsurface location, the wireline pressure setting assembly can be actuated to set the packer. Conventional wireline pressure setting assemblies employ a combustible or explosive powder charge which is actuated by means of an electric firing or triggering signal transmitted through an electric line extending to the surface of the well. Upon ignition of the powder charge contained in the well tool, the gases generated can be used to perform work in setting the well tool at the desired subsurface location.
Conventional well tools, such as bridge plugs, packers and cement retainers, employ longitudinally relatively shiftable sleeves or mandrels for use in setting the tool. By applying a downward force to one of two telescoping cylindrical members and an upward force to the other relatively telescoping cylindrical member or mandrel, an axially compressive force can be applied to anchoring slips or to sealing elements on the conventional well tool. Means can be provided to trap the initial movement of one cylindrical member relative to the other to secure the well tool in place within the well.
Conventional wireline pressure setting assemblies employ an axially shiftable piston responsive to the pressure of gases generated by the combustion or explosion of the powder charge. The movement of the piston in response to the expanding gas can then normally be transmitted by means of an appropriate connection to a sleeve in the conventional well tool. Another sleeve in the well tool can be attached to the cylindrical housing of the pressure setting assembly, and relative movement between the pressure setting piston and the pressure setting assembly housing will be transmitted to the relatively telescoping inner and outer cylindrical sleeves of the well tool and the well tool can be set.
Although the rate of combustion or explosion of the powder charge can be regulated to a certain degree, abrupt pressure changes can still result. These abrupt pressure changes could subject the actuating piston in the pressure setting assembly to rapid acceleration or jerks. One means of preventing such rapid acceleration is to provide a dash-pot assembly in which a suitable liquid, such as oil, is forced by movement of the actuating piston to flow at a relatively restricted rate through a port. Such dash-pot assemblies will therefore reduce the rate at which the actuating piston can be moved and will further increase the setting time for the tool. However, the rate of gas generation by the powder charge can be controlled only within certain limits and the generation of compressible gas to drive the piston will generally last only over a period of seconds, thus limiting the tool to fairly rapid actuation of the conventional well tool.
In some conventional wireline pressure setting assemblies, the forces generated by the expanding gases from the ignition of the powder charge is transmitted by means of a floating piston to a suitable liquid, such as oil, contained within an enclosed chamber. The forces transmitted to the incompressible liquid or oil are then transmitted to a separate piston attached to one of the cylindrical members in the conventional well tool. This separate piston has an atmospheric chamber located on its surface opposite the surface subjected to the force transmitted by the incompressible oil. The pressure differential will thus cause the piston to move to transmit a setting force to the attached well tool. A suitable dash-pot assembly may be incorporated within the enclosed chamber containing a hydraulic fluid to again act as a dash-pot, reducing acceleration of the actuating piston. It has been found that the orifice through which the incompressible fluid is shifted cannot be reduced in diameter sufficient to significantly increase the pressure setting time for conventional tools. A restriction of the orifice contained within the enclosed chamber results in a greater pressure within the chamber containing the expanding gas products of combustion or explosion of the powder charge. This increased pressure in turn results in a more rapid generation of combustion or explosion of the driving powder charge.
Although such conventional pressure setting assemblies have been successfully used in a wide variety of applications, there have been problems with conventional pressure setting assemblies used in very deep wells and at high temperatures exceeding 400.degree.-450.degree. F. At these temperatures, the powder charges or propellants, as well as ignition devices, may not ignite or burn satisfactorily. Pressure setting assemblies in which hydrostatic pressure is used to supply the actuating force have been suggested for use under these circumstances. In one device, a valve is shifted from a closed to an open position by hydrostatic pressure. Movement of the valve is possible by the disintegration of a material initially holding the valve in the closed position. When the valve is shifted to the open position, the hydrostatic pressure can act on an axially shiftable piston which transmits a setting force to the well tool in much the same manner as in more conventional powder charge driven apparatus. The pressure differential created by hydrostatic pressure acting on one surface of the actuating piston and atmospheric pressure acting on the other surface of the actuating piston is sufficient to drive the piston and to set the tool. Such tools can also be used in a tandem multi-piston configuration to multiply the total force which can be exerted by the apparatus in performing the setting operation. Although restricted flow passages can be used in the shiftable valve to reduce the flow rate of hydrostatic fluids, the degree of restriction is limited and such restricted flow passages are used to avoid the imposition of sudden or shock loads on the apparatus. In one version of such a hydrostatic pressure operated apparatus, the valve is subsequently fully opened after initially providing a restricted flow passage to minimize the initial shock loads when hydrostatic pressure is exerted upon the piston assembly. When the valve is subsequently fully open, the hydrostatic pressure acts on the actuating pistons without passing through any restricted flow passage.
In addition to the problems encountered with ignitors and powder charges at high temperatures, it is often necessary to set a packer at a much slower rate than is possible utilizing conventional pressure setting tools. For example, if a wireline packer employs a thermoplastic packing element, a relatively lengthy setting time is required. The rate of deformation of thermoplastic packing elements, such as Teflon packing element, which is necessary to establish a suitable seal, is slow relative to conventional setting times. A pressure setting assembly capable of exerting a setting force over a long period of time, of minutes or even hours rather than a few seconds as in conventional tools, is therefore highly desirable. Even with conventional packing elements, a tool set by a relatively lower force exerted over a relatively longer period of time can result in a better seal being established by the packing element. Furthermore, a pressure setting assembly capable of exerting pressure setting forces over a greater range than is currently available and for time periods varying from conventional setting times to longer periods would be highly desirable.